In sliding surfaces of a lubricated sliding pairing it is essential for the amount of the sliding friction and also for the service life of the sliding pairing, in particular of the sliding bearing that a sufficient amount of lubricant is provided in all operating conditions and in a distribution that is as even as possible between contact surfaces of the sliding bearing. In particular a beginning of the relative movement between the sliding surfaces is critical.
Through an increasing use of start-stop systems in motor vehicles this relevance substantially increases at bearings of a crank shaft since the number of start up processes of the sliding bearings is increased by a factor of 100 or more.
For this reason contact surfaces of sliding bearings are machined so that they have small indentations which serve as a reservoir for a lubricant. The indentations are provided due to the normal roughness of the material of the sliding surface or they are introduced in a controlled manner. Therefore the contact portion of a sliding bearing, thus the surface portion where the contact surfaces actually contact one another is always significantly below 100% partially even below 60%.
The respective structuring of the sliding surfaces is provided through special machining steps like grinding, finishing or honing wherein, however, a particular arrangement of the indentations cannot be predetermined and also the variation in respect to size, in particular depth of the indentations is relatively large. In particular the result of the structuring depends also greatly upon the experience of the person performing the work.
In order to obtain a defined structuring of the contact surface of a sliding bearing with respect to number, size, depth and distribution of the indentations it is also already known to impact the surface with a laser in order to obtain the desired indentations.
This method, however, on the one hand side has a disadvantage in that it is very time consuming for a large number of indentations and furthermore the impacting laser beam does not only produce an indentation in the surface but also a protrusion that envelops the indentation with a ring shape, wherein the protrusion is undesirable in many cases and requires additional finishing for removing the protrusion. Typically a flank shape of the recess produced by the laser is not controllable.
It is another disadvantage that laser machining provides a spatially limited strong heating and subsequent quick cooling which leads to undesirable new hardened zones.
Furthermore the machining method of electrochemical etching (ECM) is known which can also be used in a pulsed variant (PECM).
This produces three dimensional surfaces, for example the three dimensional surfaces of coins or the described indentations are introduced into surfaces wherein typically only a removal of 30 μm at the most is economically viable with this method.
Through approaching an accordingly configured negative electrode towards the surfaces to be machined material is removed from this surface which is possible through this method in a much finer manner than through electrical eroding.
For electrical current conduction and removal of eroded materials a current conducting liquid is pressed through a gap between the tool and the work piece during the entire process.
In crank shafts, in particular crank shafts for car engines with high numbers of cylinders, as work pieces an additional complication is that they are instable work pieces and thus difficult to machine. Dimensional compliance of a finished crank shaft is primarily determined besides by axial bearing width by determining the following parameters:                diameter deviation=maximum deviation from a predetermined nominal diameter of the bearing pinion,        circularity=macroscopic deviation from a circular nominal contour of the bearing pin stated by the distance of an outer and inner enveloping circle,        concentricity=radial dimensional deviation for a rotating work piece caused by an eccentricity of the rotating bearing and/or a dimensional deviation of the bearing shape from an ideal circular shape,        roughness in the form of a mean single depth of roughness Rz=computed value representing the microscopic roughness of a surface of a bearing,        contact surface portion=the contacting surface portion of the microscopically viewed surface structure which contacts a contacting opposite surface, and in addition for the crank bearings,        stroke deviation=dimensional deviation of the actual stroke (distance of the actual center of the crank pin from an actual center of the center bearings) from the nominal stroke, and        angular deviation=in degrees or longitudinal dimension with reference to stroke, circumferential deviation of the actual angular position of the crank pin from its target angular position relative to a center axis and with reference to the angular position relative to the remaining crank pins.        
Thus, maintaining the desired tolerances is limited for these parameters by the available machining methods and by the instability of the work piece and by the machining forces.
Also the efficiency and the economics of the method are of great importance for practical applications, in particular for mass production where cycle time and thus production costs play a significant role, wherein the machining operations for prototypes are not subject to these limitations.
This applies in particular for the last method steps when producing for example a crank shaft, fine machining and surface structuring.